Transmission control system



p 9, 1941-. K. SI NGER 2,255,683

.TRANSMISSION CONTROL SYSTEM Filed March 24, 1939 2 Sheets-Sheet 2 "aps;

Patented Sept." 9, 1941 UNITED STATES PATENT orr cr.

v TRANSMISSION CONTROL SYSTEM Kurt Sins'er, Burbank, Calif assignor toRadio Corporation of America, a corporation of Dela- 50laims.

This invention relates to current transmission systems and particularlyto the automatic control of the gain or attenuation of such systems.

Electrical current transmission systems are well known, these systemsgenerally having a linear relationship between input and output levels.It has been found desirable, however, in certain types of transmissionsystems, to be able to vary the gain or attenuation of the system. Thishas been previously accomplished both manually and automatically intransmission systems used for sound recording, radio broadcasting,telephone communications, etc., the general purpose being, in each case,to obtain compression of a certain range of input levels into a smallerrange of output levels. 7

Although the present invention relates to the automatic control of thegain of an amplifying system, it provides an improved method ofcontrolling the gain changes of the system. Specifically, it permitspredetermination of the rate of gain change and the selection of theinput level at which such a gain change will start to occur. It thusprovides greater flexibility of control than heretofore known bypermitting the gain of the amplifying system to be varied at differentrates over diflerent ranges of amplitudes.

. The present invention is directed to the method of and means forobtaining these controls in a novel and efficient manner. The startingpoint control is obtained by adiustably controlling the beginning ofoperation of a rectifier, while the control of the rate of gain changeis accomplished by adiustably varying the gain of an amplifier feedingthe rectifier.

The principal object of the invention, therefore, is to control theautomatic performance of a variable gain amplifier.

Another object of the invention is to obtain with a single amplifyingunit a multiplicity of operating characteristics. I

A further object of the invention is to obtain with asingle amplifyingunit linear amplification, volume compression, volume limiting and/orany combination thereof.

A further object of the invention is to enable the point at whichtransition from linear amplification to volume compression, or volumelimiting, and the rate at which such compression and limiting occurs, tobe predetermined.

Although the novel features which are believed to be characteristic ofthis invention are pointed out with particularity in the claims ap-Application March 24, 1939, Serial N0. 263,884

and the mode of its operation will be better understood by referring tothe following description read in conjunction with the accompanyingdrawings forming a part thereof, in which Fig. 1 is a block diagram of atype of sound recording system embodying thev invention;

Fig. 2 is a schematic circuit diagram of the invention; and

' Fig. 3 is a graph showing the relationship between input level andoutput level of a compressor unit embodying the invention.

Referring now to Fig. 1, a microphone 5 feeds a pro-amplifier 6 which,in turn, feeds a mixer I, the output of which is amplified in anamplifler 8 and then impressed on a compressor unit It. Aftercompression, the currents are amplified in an amplifier II and thenimpressed upon a recorder l2 which may be of any type known in theart,this, of course, representing only one possible use of the invention.The system Just descrlbedis the general arrangement of a recordingsystem except for the compressor l0, shown schematically in Fig. 2 andwhich will now be referred to for a detailed description of the in.vention.

The input to the circuit in Fig. 2 is at terminals I! which areconnected to the primary of a push-pull transformer it. the primariesand secondaries of which are shunted by resistances l1, l8 and i9 toprovide suitable terminations. The secondary of 'the transformer i6 isconnected to the control grids of variable mu tubes 2| and 22. Theplates of tubes 2| and 22 are coupled through a capacity-resistancenetwork comprising resistances 24, 25, 26 and 21 and condensers 28 and29, tothe grids of triode amplifier tubes 2| and 32. The plates ofamplifier tubes ti and 32 are connected to the primary of a transformer24, the secondary of which is shown connected to output terminals 35.Grid bias for the variable mu tubes 2| and 22 is obtained from thevoltage drop across a resistance 31 through which flows the plate andscreen grid currents of tubes 2| and 22 and the bleeder current of avoltage divider composed of resistances 31, 38 and 39. Resistances 38and 39 provide potential to the screen grids of tubes 2| and 22.Resistance 40 is a de-coupling resistance to prevent feedback due to aplate voltage supply common to tubes 2|, 22, 3| and 32. Condensers 44,I5, 48 and" are bypass condensers. Grid bias for tubes 3| and 32 isobtained by their plate current potential drop across a resistance 48shunted by a bypass condenser 49.

pended herewith, the manner of its organization 5 The portion of thecircuit just described is an amplifier, the gain of which is varied in amanner nowlto be described. The voltage across the secondary oftransformer 34 is impressed on a control circuit including a transformer50, the secondary of which is shunted by a potentiometer having a slider61 for varying the voltage impressed upon the grid of a triode amplifier52. Switch 63 is provided for the purpose of disconnecting the grid oftriode 52 from the potentiometer 5|. The output of the amplifier 52 isfed through a transformer 53 to the plates of a full-wave rectifier 54.Grid bias for the amplifier 52 is obtained from the voltage drop of itsplate current across a resistance 56 shunted by a condenser 51, while aresistance 53 servesas a de-coupling resistance and a condenser 58 as abypass condenser. Forobtaining an initial positive bias on the cathodesof rectifier 54 in respect to its plates, a voltage divider comprisingresistances BI, 62 and 53 is employed, resistance 62 being apotentiometerwith an adjustable slider 64 for the purpose to behereinafter described.

The rectified current flows over conductor 56 through a resistance 4|and thereby causes a voltage drop across this resistance whichconstitu'tes a negative bias applied to the control grids of variable mutubes 2| and 22. This biasing voltagevaries in accordance with thevoltage changes across the secondary of output transformer 34 and thusvaries the gain of tubes 2| and 22. Condenser 42 has a double function;

namely, it serves as a bypass condenser for the alternating currentcomponents of the rectified current and also determines, in conjunctionwith resistance 4|, the internal resistance of the rectifier 54, and theimpedance of the secondary of transformer 53, the rapidity with whichvoltage changes across resistance 4| can take place. This combination ofresistance and capacity for I the above purpose is well known in theart.

In the present invention it is possible to start variations in the gainof tubes 2| and, 22 at certain predetermined input signal levelsand-this starting point is controlled by the setting of the variablecontact 54 on resistance 52. The posi-.

signal potential applied between the plates and the cathodes ofrectifier 54, no rectified current will fiow through resistance 4| andthus no gain change'in tubes 2| and 22 will occur. As soon as the signalpotential applied to the rectifier 54 exceeds the positive cathode bias,rectified current will flow through resistance 4|, and gain variationsof tubes 2| and 22 will take place.

Resistances 6| and 53 are used, so that the adjustable voltage range ofcathode bias will be within predetermined limitations, resistance 53limiting the maximum, and resistance 6|, limiting the minimum positivebias.

It is well known that rectifiers of the type used in the invention havean inherent initial emission current, which will fiow through aresistance connected between plates and cathode even when there is nosignal potential applied to the plates. This condition wouldbe met, were1 resistance 6| omitted and slider 64 moved completely to the negativeend of potentiometer 52. This inherent initial emission current wouldthen fiow through resistance and thus produce an undesirable bias on thecontrol grids of tubes 2| and 22 and a consequent reduction of gain.This bias is undesirable because it would be subject to changes,depending upon the emission characteristics of diflerent rectifier tubesand upon emission changes due to inherent rectifier instability. The useof resistor 3| insures the. presence of a small positive bias on thecathode in respect to the plates of rectifier 54, thereby eliminatingthe possibility of current fiow through resistor 4| unless there is asignal potential applied to the plates of rectifier 54, regardless ofthe position of slider 64 on potentiometer 62. Condenser l0 bypasses theresistances 5| and 62. Plate potential for the system is impressedbetween terminals II and is of a value suitable for the type of tubesand circuit shown.

Whereas it has been described above how it is possible to control thestarting point of gain variations of tubes 2| and 22 by potentiometer82, the control of the rate of this gain variation will now beexplained. As the setting of slider 61 of potentiometer 5| editrols theamount of that signal potential will be impressed upon the grid of tube52 only when switch 68 is in its upper position as shown, and nopotential from terminals 35 will reach the tube 52 when switch 53 is inits lower position, thus eliminating compression completely and makingthe amplifier 2|, 22, 3| and 32 a conventional constant gain de vice. 1i 1 The circuit above-described, therefore, has two variable controlelements, one of which determines the point at which compression begins,or the range of compression, and the other of which determines the rateof compression. To illustrate several variations of compressioncharacteristic curves obtainable by the use of this invention, referenceis made to Fig. '3 showinga graph of the relationship between the inputlevels at terminals l5 and the output levels at terminals 35. It will benoted that the normal constant gain characteristic of the amplifier isshown by the 45 angle curve a, which condition exists when the switchlever .68 is in its lower position. For the sake of clarity, theordinates of the graph shown in Fig. 3 have been chosen for an amplifierhaving zero gain at no compression. It will also be noted that forpurposes 'of illustration the lowest input level is taken as 14 db., themaximum desirable level as 0 db., and the upper level limit as +20 db. Adifferent maximum desirable level may be chosen than the one shown inFig. 3 if the amplifier is used for broadcasting, telephonetransmission, etc., the particular maximum desirable level shown in Fig.3 being suitable for a film recording channel, as shown in Fig. l. Thefirst family of curves, illustrated by broken lines 0, d and e, is shownbeginning at a point b on curve a at a level of 12 db. It will be notedthat these curves have difierent slopes. The point b or breakaway"pointis determined by the setting of slider 64, which determines whenthe rectifier settings of slider 61 varying the input level to Beginningat point i on curve a, a second family of three curves, illustrated bydot-and-dash lines I, g and his shown, the point i again beingdetermined by a certain setting of slider 64, and the slopes of thecurves by certain settings of the slider 61. In this case, curve I showsthat an input range of 26 db. is compressed into an output range of 6db.; curve g, a range of 16 db. into 6 db.; and curve It, a range of 10db. into 6 db.; compression beginning in each case at -6 db. Althoughonly two breakaway points b and i and three curves from each point havebeen illustrated, it will be realized that substantially any breakawaypoint along curve a, as well as many rates of compression, areobtainable by different adjustments of sliders G4 and 61. Thus, a veryflexible control of the compression characteristic of an amplifier isprovided.

Referring again to Fig. 3, a full-line curve It is shown starting at abreakaway point 9' on termining the point at which said rectifier variesthe gain of said amplifier comprising a variable potentiometer having afixed section and a variable section, said fixed section preventing thefiow of initial inherent emission current in said rectifierat no signalinput.

2. An amplifying system comprising a pair of variable mu tubes connectedin push-pull, said curve a, this curve k illustrating that a range ofinput level changes amounting to 22 db. are compressed into a smallrange of output level changes amounting to 2 db., compression beginningat 2 db. A device which has this characteristic is commonly known as avolume limiter, while a device having any one of the characteristicsshown by curves 0, d, e, f, g and h is known as a volume compressor. Itis, therefore, possible by a change of settings of either one or both ofpotentiometers 5| and 62 to obtain, with a single device, volumecompressor or volume limiter characteristics.

Although the above controls are shown controlling the breakaway pointsand the rates of compression, it is to be understood that these controlsare also applicable to an amplifier used for expansion wherein a certainrange of input levels is expanded to a greater range of output levels.It is also to be understood that the amplifier-rectifier combination,which is shown connected to the output terminals 35 on Fig. 2,

could be connected to the input terminals IS instead, to produce similarresults.

I claim? tubes having normally a constant gain characteristic, a secondamplifier having a constant gain characteristic, a high potential sourcefor the anodes for said tubes, means for connecting the output of saidpush-pull tubes to the input of said second amplifier, a rectifierconnected to the output 01' said second amplifier, means for connectingsaid rectifier to said push-pull tubes, the output of said rectifiervarying the gain characteristic of said push-pull tubes, a combinationfixed and adiusable potentiometer connected across said potential sourcefor selectively determining the point at which said rectifier begins tovary the gain or said push-pull tubes, 8. variable potentiometerconnected in the input circuit of said second amplifier for selectivelydetermining the rate of variation of the gain of said push-pull tubes bysaid rectifier and means for eliminating at will the operation of saidsecond amplifier.

3. An amplifying system comprising a pair oi! 'variable mu tubesconnected in push-pull, said tubes having normally a constant gaincharacteristic, a second amplifier having a constant gaincharacteristic, a potential source for the anodes for said tubes, meansfor connecting the output of said push-pull tubes to the input of saidsecond amplifier, a rectifier connected to the output of said secondamplifier, means for connecting said rectifier to said push-pull tubes,the output of said rectifier varying the gain characteristic of saidpush-pull tubes, a combination fixed and adjustable potentiometercon- 1. An electrical current transiriission system having inputterminals and output terminals, an electrical current amplifierintermediate said input and output terminals, said amplifier having anormally constant gain, a second amplifier connected to the outputofsaid first-mentioned amplifier and having a constant gain, a potentialsource for the anodes of said amplifiers, a

rectifier connected to said second amplifier,

means for connecting the output of said rectifier to saidfirst-mentioned amplifier, the outnected across said potential sourcefor selectively determining the point at which said rectifier begins tovary the gain of said push-pull tubes, a variable potentiometerconnected in the input circuit of said second amplifierfor selectivelydetermining the rate of variation of the gain of said push-pull tubes bysaid rectifier, and means for eliminating at will the operation of saidsecond amplifier, one fixed section of said potentiometer forselectively determining the point at which said rectifier begins to varythe gain of said push-pull tubes maintaining said rectifier negativelybiased to prevent the flow of initial inherent emission current in saidrectifier at times of no signal input thereto while another fixedsection of said potentiometer limits the application of positive biasapplicable to said rectifier.

4. An electrical current transmission system variable betweenpredetermined limits, and fixed 'means in the biasing circuit of saidrectifier for preventing the flow of initial inherent emission currentin said rectifier at times of no input to said rectifier, saidpotentimeter determiningthe amount 01' positive bias potentialimpressable upon said rectifier.

5. An electrical compressor circuit comprising a normally constant gainamplifier having input. output and biasing circuits, a second constantgain amplifier connected to the output of said first-mentionedamplifier, a rectifier connected intermediate the output of said secondamplifier andthe input oi. said first amplifier, the input level to saidsecond amplifier determining the azuaoss 't gain oi said first-mentionedamplifier over a predetermined range of input levels to said secondamplifier, a potential sourcev for the anodes of all of said amplifiers,and means i'or obtaining from said potential source a biasing potentialfor said rectifier, said last-mentioned means including a potentiometerconnected across said potential source, said potentiometer having anadjustable portion to vary the bias potential on said rectifier, a fixedportion to limit the minimum bias potential applicable to said rectifierand a second fixed portion to limit the maximum bias potentialapplicable to said rectifier.

KURT SINGER.

